The semiconductor integrated circuit industry has experienced rapid growth in the past several decades. Technological advances in semiconductor materials and design have produced increasingly smaller and more complex circuits. These material and design advances have been made possible as the technologies related to processing and manufacturing have also undergone technical advances. In the course of semiconductor evolution, the number of interconnected devices per unit of area has increased as the size of the smallest component that can be reliably created has decreased.
Semiconductor fabrication relies heavily on the process of photolithography, in which light of a given frequency is used to transfer a desired pattern onto a wafer undergoing semiconductor processing. To transfer the pattern onto the wafer, a photomask (also referred to as a mask or reticle) is often used. The photomask permits and prevents light in a desired pattern onto a layer of the wafer, such as a photoresist (PR) layer, which chemically reacts to the light exposure, removing some portions of the PR and leaving other portions. The remaining PR is then used to pattern an underlying layer. As feature sizes have decreased, the wavelength of light used in photolithography to pattern layers has decreased as well, creating additional difficulties and necessitating technological advances such as the use of extreme ultraviolet (EUV) as a light source, phase-shifting masks, and other advances. Improving photomasks is important to the continued advances in the industry, because various kinds of photomask defects can result in compounding defects in the patterned layers. Photomask improvements include improves made in substrate material and quality, such as the use of low thermal expansion material (LTEM) substrates for EUV masks.
For example, during photomask fabrication, attempts are often made to clean the photomask substrate before additional layers are applied. The processes used to clean and polish the photomask substrate have improved. However, defects are often introduced during and may remain after the cleaning and polishing. Therefore, there is a continuing need for systems and techniques to improve the characteristics of mask substrates.
The various features disclosed in the drawings briefly described above will become more apparent to one of skill in the art upon reading the detailed description below.